Lower respiratory tract infections are a leading cause of death worldwide and can be complicated by the acute respiratory distress syndrome (ARDS), which is a major cause of morbidity and mortality in critically ill patients. Veterans are at increased risk of mortality from pneumonia so identifying novel mechanisms of protection may help improve veteran outcomes. Platelet deficiency, or thrombocytopenia, has consistently been associated with increased mortality in patients with severe pneumonia and ARDS. However, the mechanisms by which platelets may protect the host are poorly understood. We have recently shown a role for platelets in limiting alveolar- capillary barrier disruption and lung injury during acute Pseudomonas aeruginosa (PA) pneumonia in mice. We further showed that PA cell-free supernatant was sufficient to induce lung epithelial cell death with features of apoptosis as well as severe lung injury. Finally, we showed that released platelet factors can limit apoptotic cell death in lung epithelial cells in vitro as well as limit cell death and lung injury during acute PA infection in thrombocytopenic mice. However, it remains unclear the mechanisms by which platelet factors contribute to lung epithelial cyto-protection during PA infection. The main objective of this proposal is to investigate the mechanisms and implications of infection-triggered lung epithelial cell death as well as the mechanisms by which platelets and their factors provide lung epithelial cyto-protection during PA pneumonia and lung injury. Three aims will be studied. Aim 1 will investigate whether PA supernatant induces lung epithelial mitochondrial damage that leads to initiation of cell death pathways, whether the intrinsic pathway of apoptosis that classically follows mitochondrial damage contributes to PA-triggered lung epithelial cell death in vivo, and whether inhibition of lung cell death limits disruption of the alveolar-capillary barrier during PA infection in both wildtype and platelet deficient mice. Aim 2 will investigate whether platelet factors modulate post-translational anti-apoptotic pathways, examine the transcriptional profile of lung epithelial cells after PA-mediated injury in the presence or absence of platelet releasate, and survey the role of candidate platelet proteins in providing lung epithelial cyto- protection. Aim 3 will utilize development of a novel thrombocytopenic type 2 lung epithelial reporter mouse to quantify the survival and proliferation of alveolar epithelial cells to determine whether platelets are required for optimal lung epithelial repair after injury. We will also expand the generalizability of our findings by investigating the role of platelets in protecting the lung during Staphylococcus aureus pneumonia, which is similar to PA in its ability to mediate lung epithelial damage with secreted toxins. By improving understanding of the mechanisms by which platelets may provide protection during severe pneumonia, this project may help to provide rational therapeutic strategies to improve morbidity and mortality of veterans and other critically ill patients. Furthermore, this project will provide the applicant the opportunity to develop a scientific toolkit and academic portfolio to support the transition to independence as a physician-scientist focused on veteran-centered clinical and scientific research issues with the long-term career goal to become a leader in the Veterans Health Administration and academic pulmonary medicine. The applicant?s research goal is to improve understanding of how the lung interacts with and employs cellular and humoral elements of innate immunity to combat pathogens and manage injury with the potential for translational applications in treatment of severe pneumonia and acute lung injury for veterans and other patients.